1. Field of the Invention
The present invention relates to a pressure contact type semiconductor device, and particularly, to such a semiconductor device in which a distortion buffer plate and a semiconductor body are in an alloy-free contact to each other.
2. Description of the Background Art
Among power pressure contact type semiconductor devices (i.e., pressure contact power devices), those in which a semiconductor body is disposed without brazing (alloy-free) are known as alloy-free pressure contact type semiconductor devices. Where a distortion buffer plate and a semiconductor body are adjoined to each other using a brazing material such as a solder and aluminum, large thermal stress will be created due to a difference in expansion coefficients. To avoid this, in an alloy-free pressure contact type semiconductor device, a semiconductor body is sandwiched by electrodes from the top and the bottom through distortion buffer plates made of molybdenum or the like. Due to this structure, in this type of semiconductor device, electric connection is realized only by a pressure contact during use of the semiconductor device.
FIG. 14 is a cross-sectional view of a conventional power diode which is structured as an alloy-free pressure contact type semiconductor device. In the pressure contact type semiconductor device of FIG. 14, a first and a second disk-shaped distortion buffer plates 2A and 2K are disposed on a top and a bottom major surfaces of a semiconductor body 1, respectively. The distortion buffer plates 2A and 2K are in an alloy-free contact to an anode electrode layer and a cathode electrode layer (both not shown) which are formed in the top and the bottom major surfaces, respectively, of a semiconductor substrate which forms the semiconductor body 1.
A protector 3 is fixed to a periphery of the semiconductor body 1 to prevent electric discharge from the periphery of the semiconductor body 1 and to protect the periphery of the semiconductor body 1.
A disk-shaped anode electrode plate 4A and a disk-shaped cathode electrode plate 4K are disposed on the distortion buffer plates 2A and 2K, respectively, in an alloy-free contact.
This stacked structure is housed in a cylindrical ceramic casing 7. Through a metallic anode flange 5A and a metallic cathode flange 5K, respectively, the base portions of the anode electrode plate 4A and the cathode electrode plate 4K are linked to the casing 7.
During use of such a pressure contact type semiconductor device in a predetermined apparatus, the anode flange 5A and the cathode flange 5K are subjected to external force F of the direction indicated by the arrows in FIG. 14 so as to be pressed respectively through the distortion buffer plates 2A and 2K and to be securely electrically connected to the top and the bottom major surfaces of the semiconductor body 1.
In general, the cathode flange 5K is brazed to the casing 7 while the anode flange 5A is welded or cold-welded to a flange 5F which is attached to the casing 7 in advance. The cathode flange 5K and the anode flange 5A not only seal the casing 7 airtight but also function as springs.
The reason of using the cathode flange 5K and the anode flange 5A as springs as well is to absorb thermal stress which is created when the anode flange 5A and the cathode flange 5K are brazed or welded to the casing 7 due to a difference in expansion coefficients, and to absorb a tensile stress which is created at joints of the casing 7 and the anode flange 5A and the cathode flange 5K when the structural members contained in the casing 7 have different thicknesses and the overall height of the assembled structure is different from what is originally expected.
Briefly summarizing, the cathode flange 5K and the anode flange 5A must have a sufficient spring strength so that:
a) thermal stress due to a difference in the expansion coefficients is absorbed during fixing of the flanges;
b) the overall spring strength of the flanges does not have a resonance point against external vibration; and
c) tensile stress is absorbed which is created at the flange joints due to the different thicknesses of the structural members contained in the casing 7.
The spring strength which satisfies the conditions above is not as large as the external force F which is applied to the anode flange 5A and the cathode flange 5K during pressing. Under no pressure (i.e., in which the external force F is not applied), the distortion buffer plates 2A and 2K and the semiconductor body 1 sandwiched by the anode flange 5A and the cathode flange 5K are not constrained completely.
If the pressure contact type semiconductor device in such an under-no-pressure condition is heated, for example, left for six hours at 150.degree. C. for the purpose of aging, since the casing 7 is sealed airtight, pressure mounts within the casing 7. Eventually, the anode electrode plate 4A and the cathode electrode plate 4K are pushed upward from the flange joints, cracking gaps between the structural members which are contained in the casing 7.
If the structural members contained in the casing 7 have different thicknesses, gaps exist between some structural members even when the device is placed at a normal temperature.
In the conventional pressure contact type semiconductor device having such a structure, if gaps are created between the semiconductor body 1 and the distortion buffer plates 2A and 2K, external vibration during assembling or transportation allows the semiconductor body 1 and the distortion buffer plates 2A and 2K to vibrate independently of each other. Because of this, the semiconductor body 1 is subjected to an offset load of an axial direction of the semiconductor body 1.
The gaps also reduce friction between the semiconductor body 1 and the distortion buffer plates 2A and 2K at the respective contact surfaces. This permits the distortion buffer plates 2A and 2K to vibrate in a radial direction as well. As a result, the protector 3 falls off from the periphery of the semiconductor substrate. Other possibility is that the offset load acting upon the semiconductor body 1 through the protector 3 cracks the semiconductor substrate of the semiconductor body 1, which degrades the reliability of the semiconductor device.
To fix the components which are housed in the casing 7 and to thereby prevent gaps created between the components, the conventional pressure contact type semiconductor device needs be transported while externally pressed using a pressure contact tool such as a simplified stack. Thus, the conventional pressure contact type semiconductor device is not convenient for transportation and is costly.